The overall objective of this proposed research is to advance the ability of near-IR (NIR) imaging to assess the depth and severity of carious lesions (dental decay). If diagnosed during early stages in the disease process, caries progression can be arrested/remineralized by non-surgical means through fluoride therapy, anti-bacterial therapy, or by low intensity laser irradiation. Clinicians need new imaging technologies that function using non- ionizing light and that aid in caries diagnosis and management/intervention by reliably tracking the pathology of carious lesions, from their initial stages until cavitation, n order to determine if the lesion is actively expanding or has been arrested. Light scattering in sound enamel and dentin is the principal factor limiting optical transmission through the tooth in the visible range from 400-700-nm. Light scattering in enamel decreases as the wavelength increases (1/?3), where ? represents the wavelength, due to the size of the principal light scatters1,3. The central hypothesis motivating this application is that the near-infrared region from 1300- 1700-nm holds the greatest potential for new optical imaging technologies due to markedly increased optical transmission and multispectral differences in water absorption. Previous in vitro and in vivo studies have demonstrated that NIR reflectance and transillumination images work best using different optimal wavelengths within the NIR spectral region, due to varying attenuation from water absorption and endogenous differences in water content among dental hard tissues4-6. The overall objectives of this application will be achieved through the following specific aims. (1) To test the hypothesis that multispectral NIR reflectance and transillumination measurements of occlusal carious lesions can be combined for improved assessment of lesion depth and severity. (2) To test the hypothesis that multispectral NIR reflectance measurements can be used to detect interproximal carious lesions viewed from the occlusal surface. The successful completion of this project is likely to substantially advance clinical dentistry by providing the necessary technology to detect early stages of caries and quantitatively monitor their progression. Furthermore, because there are no health risks from imaging with NIR light, and this technology can be miniaturized into NIR imaging probes at low cost, NIR imaging has the potential to be a universal caries screening method used globally and routinely to detect dental decay.